docs: def check_relations(self, relations):
          """Recursive function which checks if a relation is valid."""
          for rel in relations:
              if not rel:
                  continue
              fields = rel.split('.', 1)
  
              local_field = fields[0]
              if local_field not in self.fields:
                  raise ValueError('Unknown field "{}"'.format(local_field))
  
              field = self.fields[local_field]
              if not isinstance(field, BaseRelationship):
                  raise ValueError('Can only include relationships. "{}" is a "{}"'
                                   .format(field.name, field.__class__.__name__))
  
              field.include_data = True
              if len(fields) > 1:
                  field.schema.check_relations(fields[1:])